In grades 4 through about 7, there isn’t a whole lot of radically new math material that a student is expected to learn. They will see more and more sophisticated versions of the skills as they advance in years, but nothing that’s that big of a departure from what they’ve already been shown beginning as early as 4th grade. So basically, student’s have FOUR YEARS to get basic math down.
Then along comes Algebra. Even with initiatives to get students thinking algebraically well before they take an actual algebra course, still an Algebra 1 course represents a radically new set of skills. And rather than having multiple years to get it down, students have one single short fast semester, with the next 3+ years of their math career riding on it.